The present invention relates generally to apparatuses and methods for magnetically confining molten metal and more particularly to an apparatus and method for preventing the escape of molten metal through the open side of a vertically extending gap between two horizontally spaced members and within which the molten metal is located.
An example of an environment in which the present invention is intended to operate is an arrangement for continuously casting molten metal directly into strip, e.g. steel strip. Such an apparatus typically comprises a pair of horizontally spaced rolls mounted for rotation in opposite rotational senses about respective horizontal axes. The two rolls define a horizontally disposed, vertically extending gap therebetween for receiving the molten metal. The gap defined by the rolls tapers in a downward direction. The rolls are cooled, and in turn cool the molten metal as the molten metal descends through the gap.
The gap has horizontally spaced, open opposite ends adjacent the ends of the two rolls. The molten metal is unconfined by the rolls at the open ends of the gap. To prevent molten metal from escaping outwardly through the open ends of the gap, mechanical dams or seals have been employed.
Mechanical dams have drawbacks because the dam is in physical contact with both the rotating rolls and the molten metal. As a result, the dam is subject to wear, leaking and breakage and can cause freezing and large thermal gradients in the molten metal. Moreover, contact between the mechanical dam and the solidifying metal can cause irregularities along the edges of metal strip cast in this manner, thereby offsetting the advantages of continuous casting over the conventional method of rolling metal strip from a thicker, solid entity.
The advantages obtained from the continuous casting of metal strip, and the disadvantages arising from the use of mechanical dams or seals are described in more detail in Praeg U.S. Pat. No. 4,936,374 and in Lari et al U.S. Pat. No. 4,974,661, and the disclosures of each of these patents are incorporated herein by reference.
To overcome the disadvantages inherent in the employment of mechanical dams or seals, efforts have been made to contain the molten metal at the open end of the gap between the rolls by employing an electromagnet having a core encircled by a conductive coil through which an alternating electric current flows and having a pair of magnet poles located adjacent the open end of the gap. The magnet is energized by the flow of alternating current through the coil, and the magnet generates an alternating or time-varying magnetic field extending across the open end of the gap between the poles of the magnet. The magnetic field can be either horizontally disposed or vertically disposed, depending upon the disposition of the poles of the magnet. Examples of magnets which produce a horizontal field are described in the aforementioned Praeg U.S. Pat. No. 4,936,374; and examples of magnets which produce a vertical magnetic field are described in the aforementioned Lari et al U.S. Pat. No. 4,974,661.
The alternating magnetic field induces eddy currents in the molten metal adjacent the open end of the gap, creating a repulsive force which urges the molten metal away from the magnetic field generated by the magnet and thus away from the open end of the gap.
The static pressure force urging the molten metal outwardly through the open end of the gap between the rolls increases with increased depth of the molten metal, and the magnetic pressure exerted by the alternating magnetic field must be sufficient to counter the maximum outward pressure exerted on the molten metal. A more detailed discussion of the considerations described in the preceding sentence and of the various parameters involved in those considerations are contained in the aforementioned Praeg and Lari et al. U.S. Patents.
Another expedient for containing molten metal at the open end of a gap between a pair of members is to locate, adjacent the open end of the gap, a coil through which an alternating current flows. This causes the coil to generate a magnetic field which induces eddy currents in the molten metal adjacent the open end of the gap resulting in a repulsive force similar to that described above in connection with the magnetic field generated by an electromagnet. Embodiments of this type of expedient are described in Olsson U.S. Pat. No. 4,020,890, and the disclosure therein is incorporated herein by reference.
The use of a coil to directly generate the magnetic field adjacent the open end of the gap is more efficient than the use of an electromagnet because, when employing an electromagnet, the coil is used to energize the core of a magnet through which magnetic flux must travel to the magnet poles which then generate a magnetic field adjacent the open end of the gap. As a result, there is so-called "core loss" when a coil is employed to energize an electromagnet; but core loss is not a significant factor when the coil is employed to directly generate the magnetic field at the open end of the gap.
A drawback to the latter expedient is that the coil must be placed quite close to the open end of the gap in order to generate a magnetic field which will contain the molten metal there. In the expedient employing an electromagnet, the coil can be relatively remote from the open end of the gap. The closer the coil is to the molten steel, the more severe the thermal conditions to which the coil is subjected. Another drawback to the expedient employing a coil for directly generating the magnetic field at the open end of the gap is that part of the magnetic field is radiated in a direction away from the open end of the gap, thereby decreasing the efficiency of the coil. The problem described in the preceding sentence can also be a problem when employing any electromagnet.